When I saw hat you were going to use 5kHz frequency shift, I was immediately concerned, since the broadcast FM deviation is 75kHz, so you will get a very small output signal. Also, it sounded like you plan to transmit a 10 or 15kHz tone frequency-modulated on a 75MHz carrier. This will not give you signals at 75.010 and 75.015 (FSK), except at very particular Modulation Indexes. Single- or double-sideband suppressed-carrier modulation will do that, but is more complex. If you are just detecting the received 10 and 15kHz tones, however, you should FM modulate them at a 50 to 75kHz deviation, for best received signal level.

As for antenna matching: while I am not volunteering to donate the necessary equipment for that, antenna analyzers are available for the Ham Radio market, from MFJ and others. Prices are in the $280 range.

Thanks for your thoughts. First, 75MHz does propagate well under water. Though, it has to be fresh water and not salt water.

As to the method of post processing the data, it is looking like a comparator may be the way to go. I have found some more information lately that seems to show that this may work. I have a post that should go live here in a day or so on this.

For your digital receiver (which is too much for your application) the only issue is that the standard frequency deviation for an FM receiver is 50khz (old school). 5khz could be too small. (The bigger the frequency deviation the better coverage). Otherwise, you should get the data that you need.

The below data refers to an analog FM receiver.

In some conditions a simple comparator at the frequency detector output could be enough.

But if your transmitter should transmitt long zeros, and long 1s, than the receiver should be able to output them. This means that the receiver should be able to output DC. Otherwise, before generating the FSK, you have to process your digital signal, to be able to transmit only zero's or only 1's. For example, a Manchester coder/decoder should be used.

or, the 3rd sollution is to use an analogue IC FSK receiver (Motorola), or just to buy a FSK receiver module for 75Mhz band.

I do not know anything about 75Mhz propagation under the water. Is it possible?

I'm glad I could contribute. As for the video idea, I am aware of the BW limitations. I was just kidding around. That's why I quipped about fun being had by all, because it wouldn't be had by you as you were trying to clime that little Mt. Everest of a problem.

When I played around with Görtzel vs Fourier on an stm32f4 for vowel detection it was about four times faster than a classic FFT. Also, the code was less complex. Again, it all depends on the application but it seems to me you could benefit from it.

Kevin, I appreciate your thoughts. Even your initial ideas are valuable as it helps me understand more about some ideas when dealing with a signal in which you have both the I and Q. This is an area that I am really trying to dig into and learn more. It takes a bit longer when you do not have a local expert to help, but little by little I am coming to a better understanding of RF and DSP concepts.

One of the next projects that I would like to do is to create a full receiver. Front end and all. Potentially use it as a learning project for FPGA's. I would do it as a directo conversion receiver with a delta sigma converter, and target a frequency range in the 100-200kHz range. It is just a concept right now.

Yeah, those are some good ideas, but it does get a bit easier than that. You can actually set ut up so that the receiver looks at each frame of data and determins if it is good or not. If you have a certain period of time where you do not get good data, you have an automatice action programmed into the receiver that commands the boat to the surface. This is what you are getting at with your second idea. I do like though, the concept of initiating not only a surfacing command, but also to circle. That is an interesting idea. I will have to think about that one.

The only problem with the video is that it requires a lot of bandwidth and the frequencies that might be used (900MHz or 2.4GHz) all attenuate too significantly to use. The only other option might to be see if I can transmit in an amature frequency band in the 50MHz region (I forget exactly what the band limits are), but I am not sure how much bandwidth would be needed for an ok sized picture.

At least as far as losing the boat to depth is concerned, I would think that one could employ a pressure sensor of some sort, for depth detection, that would initiate a local override command to automatically actuate an ascent. That would be a snap for a Senior Mechanical Engineer, no?

Now, as for losing the boat to distance that get's bit trickier. Perhaps if it doesn't receive a viable command after x seconds/minutes a more sophisticated local overide command goes into effect: ascend and circle.

Then after you've nailed down das but's mechanical behavior, you can start working on how that nifty littel radio chip could transmit real-time video from the little camera you'll be mounting to the bow. Much fun will be had by all.

Yeah. As to rolling your own, there is no issue, it is not an intentional transmitter. I already consulted with the FCC on this. Even for transmitters, as I understand the rules, I can build up to 5 of a design for personal use as long as the follow good engineering practice.

In practical terms for transmitters, this means that as long as I do a good job, and I do not cause complaints because I have a lot of spurrious signals interfering with other licensed band users, I am most likely ok. This by far is not to be considered an official legal interpretations of that for transmitters, though.